Aspects of scalar dynamics and vacuum energy in the string swampland program

Resumen

This thesis, written as a compendium of articles, investigates the properties of scalar dynamics and vacuum energy in the context of string theory compactifications, and their connection with the swampland program. In the first article presented in the thesis, we consider warped throats with locally AdS geometry, and study their stability properties. Motivated by considering the near horizon limit of systems of fractional D-branes at singularities, we propose that such backgrounds cannot be stable in the absence of supersymmetry, and thus generalize the swampland criterion that forbids stable non supersymmetric AdS vacua. This allows us to rule out large classes of warped throats with supersymmetry breaking ingredients, shedding new light on already known instabilities, including the runaway in the dP1 theory, and unveiling novel decay mechanisms such as the one associated to N = 2 fractional branes. In the second article we focus on the asymptotic Klebanov–Tseytlin solution, regarded as a compactification to five dimensions in which an axion runs in the radial direction of the locally AdS spacetime. The model can be reinterpreted as a fully backreacted solution of transplanckian axion monodromy, with the axion traversing arbitrarily large distances in field space, and provides an existence proof of transplanckian field excursions in string theory. In particular, we discuss how the ten-dimensional solution fully encodes the backreaction of the axion dynamics including the impact on the axion kinetic term, and the backreaction on other sectors, such as the compactification moduli and the vacuum energy. In the third article we propose a refinement of certain swampland conjectures in the presence of discrete gauge symmetries. We consider theories with both discrete and continuous gauge symmetries, and relate the gauge coupling of the continuous symmetry with the order of the discrete symmetry. We also study discrete symmetries associated to domain walls, and we use them to justify the presence of separation of scales in an infinite family of AdS4 flux vacua of type IIA string theory. In the last two articles of the thesis, we study running solutions sourced by tadpoles for dynamical fields, and analyse their properties in large classes of string theory models. These solutions can only extend up to a finite distance in spacetime, scaling inversely with the strength of the tadpole, and are capped off by cobordism walls of nothing in a dynamical realization of the cobordism conjecture. We also discuss domain walls interpolating between different (but cobordant) theories. The key criterion to distinguish between the two kinds of walls is related to the distance in field space, and suggests a connection with the distance conjecture